1. Field of the Invention
The present invention relates to a power supply switching circuit, and more particularly to a power supply switching circuit for a Universal Serial Bus (USB) circuit.
2. General Background
A USB circuit of a contemporary motherboard generally has a system operating voltage of 12 volts, and a stand-by voltage of 5 volts. With the changes of the working status of the motherboard, the USB circuit is switched between the two voltages. A switching circuit receives a “Power Good (PG)” signal from the motherboard which indicates the working status of the motherboard, and then outputs a Power_OK (PWR_OK) signal.
Currently, chips including Intel's 810, 815, 845, 850, 875, 915, 925, etc. series all adopt Metal-Oxide Semiconductor Field Effect Transistors (MOSFETs) as controlling switches in the USB circuits.
Referring to FIG. 4, a conventional power supply switching circuit generally includes two MOSFETs 1, 3. A gate of the MOSFET 1 is connected to an input terminal 2 to receive a PG signal. A drain of the MOSFET 1 is connected to a gate of the MOSFET 3, and is also connected a stand-by power supply terminal 10 via a drain resistor 6. The stand-by power supply terminal 10 supplies a voltage of 5 volts. A source of the MOSFET 1 is grounded. A drain of the MOSFET 3 is connected to a system power supply terminal 12 via a drain resistor 8. The system power supply terminal 12 supplies a voltage of 12 volts. A source of the MOSFET 3 is grounded. The drain of the MOSFET 3 also acts as a signal output terminal 4 to output a PWR_OK signal. A threshold voltage of the MOSFET 1 or 3 is 2.2 volts. When the input terminal 2 receives a logic high PG signal which is higher than 2.2 volts, the MOSFET 1 is turned on (i.e., is at a working status). At this time, the drain voltage of the MOSFET 1 drops from the 5 volt stand-by voltage to a lower level. When the drain voltage of the MOSFET 1 drops to a lower level which is less than 2.2 volts, the MOSFET 3 is turned off (i.e., is at a non-working status). The drain voltage of the MOSFET 3 is raised to 12 volts. Thus, the output terminal 4 outputs a 12 volt PWR_OK signal. The 12 volt PWR_OK signal switches the USB circuit to an operating voltage. Conversely, when the input terminal 2 receives a logic low PG signal which is lower than 2.2 volts, the MOSFET 1 is turned off. The drain voltage of the MOSFET 1 is raised to 5 volts, the MOSFET 3 is thereby turned on. The output terminal 4 outputs a zero volt PWR_OK signal, which switches the USB circuit to a stand-by voltage.
Referring also to FIG. 5, a signal line 9′ indicates a voltage signal at a node 9 of the switching circuit. The node 9 corresponds to the drain of the MOSFET 1. A signal line 4′ indicates the PWR_OK signal of the output terminal 4. When the voltage at the node 9 drops from 5 volts to 2.2 volts, the voltage at the output terminal 4 is turned over from zero volts to 12 volts. A turnover voltage of the switching circuit is 2.2 volts, which is close to a 2.4 volt certain logic high voltage. If other factors, such as ambient temperature fluctuations and unstable voltages, interfere with the MOSFETs 1, 3, the MOSFETs 1, 3 may not be turned over, even if the input PG signal reaches the logic high voltage. This can lower the performance of the switching circuit.
In addition, even though the switching circuit performs the USB circuit power supply switching, the MOSFETs 1, 3 are generally quite expensive, which increases the cost of the USB circuit. Furthermore, the threshold voltage of the MOSFETs is close to a lower limit (2.4 volts) of the logic high voltage of the PG signal. This can lead to instability of the switching circuit.
What is needed is a power supply switching circuit for a USB circuit which is relatively inexpensive and which has optimized performance.